The sample collection unit of a microsampler puncturing system is not only used for generating a piercing wound, but rather also for collecting a small sample of body fluid. The collected sample volume is typically at most a few microliters. Therefore puncturing systems of this type are referred to as “microsampler puncturing systems”. A sample collection unit for a microsampler puncturing system is known from WO 03/009759 A1. For sample collection, the piercing element, after a first piercing, by which a piercing channel is generated, may be pierced again at a lesser depth into the piercing channel in a second piercing (European Patent Application 05019190.7). The piercing element remains in the piercing channel during a collecting phase after the second piercing, so that body fluid may be withdrawn through the capillary channel. A further possibility for sample collection is to only partially retract the piercing element after the piercing for generating the piercing channel, so that it remains at a lesser depth in the piercing channel during a collecting phase (WO 02/100275).
Due to the automatic sample collection, microsampler puncturing systems are a great convenience for diabetics, who must check their blood sugar level a plurality of times per day. An object of the invention is to indicate a way in which microsampler puncturing systems may be improved further with respect to sample acquisition and pain perception.
This object is achieved by a microsampler puncturing system for collecting a body fluid sample from a body part, comprising a sample collection unit having a piercing element, and a puncturing instrument having a drive, by which the sample collection unit is movable on a movement path for piercing the piercing element into the skin of the body part and retract it again by a piercing and retraction movement, a setting device for setting a defined puncturing depth (dm) of the piercing wound to be generated, and a control device for controlling the piercing and retraction movement having the following sequentially executed movement phases
a forward phase, in which the piercing element is moved in a puncturing direction and pierced to a defined puncturing depth,
a retraction phase, in which the piercing element is partially retracted by a retraction distance and decelerated toward the end of the retraction phase (R1), so that it projects into the skin at a defined residual puncturing depth, and
a collecting phase, in which the piercing element projects into the skin at the defined residual puncturing depth and a body fluid sample is withdrawn by the sample acquisition unit,
the setting device being adapted for setting the puncturing depth independently of the mean value, with reference to the collecting phase, of the defined residual puncturing depth.
The residual puncturing depth does not have to be constant during the collecting phase. Rather, a slow movement during the collecting phase, preferably at a velocity of at most 0.3 mm/seconds, more preferably at most 0.1 mm/seconds, is possible and in many applications even advantageous. In particular, this is a slow retraction movement (opposite to the puncturing direction). In order to take this into consideration in the definition of the invention, reference is not made to the residual puncturing depth dr during the collecting phase, but rather to its mean value <dr>. The mean value <dr> of the residual puncturing depth dr is understood as the quotient of the time integral of the puncturing depth d(t) from the beginning of the collecting phase at the time t1 up to the end of the collecting phase at the time t2 and the duration of the collecting phase (t2 minus t1):
  <  dr  >=                    ∫                  t          1                          t          2                    ⁢                        ⅆ                      (            t            )                          ⁢                  ⅆ          t                                    t        2            -              t        1            
In general, the beginning and end of the collecting phase, i.e., the times t1 and t2 are defined in that the withdrawal of the body fluid, after the deceleration of the piercing element, starts and ends, respectively.
The piercing element is preferably stopped at the end of the retraction phase, so that the residual puncturing depth is constant during the collecting phase. In this case, the mean value <dr> corresponds to the constant value of the residual puncturing depth.
In the context of the invention it has been established that the residual puncturing depth of the piercing element during the collecting phase is very important for the sample acquisition and the pain perception. In particular, it is important for low-pain and efficient sample collection that a defined value of the puncturing depth can be set independently of the predetermined (defined) value of the residual puncturing depth. In other words, the value of the defined residual puncturing depth should not be adjusted automatically by the same amount when the setting of the defined puncturing depth is changed. In this respect, the invention differs significantly from previously known microsampler puncturing systems:
Typically, in known microsampler puncturing systems, the residual puncturing depth was changed congruently (i.e., in the same direction and by the same absolute value) when the setting of the puncturing depth was changed. This corresponds to the prevailing opinion that, if a person requires a greater puncturing depth for obtaining blood (for example, a worker having thick callous), the residual puncturing depth must also be correspondingly greater.
In exceptional cases, only the puncturing depth was set in a defined manner, while with respect to the residual puncturing depth, the design was not adapted to ensure a predetermined (defined) depth value. For example, in US 2004/0059256, a puncturing system is described in which the residual puncturing depth is a function of the friction of several components and of the orientation of the puncturing instrument during use. This corresponds to the opinion that it is not the residual puncturing depth, but rather the puncturing depth which is important with respect to low-pain acquisition of a sufficiently large blood droplet.
Surprisingly, setting the residual puncturing depth to adapt to the requirements of a specific patient is not absolutely necessary, rather a defined, non-settable residual puncturing depth may be used independently of the patient, although thickness and strength of the uppermost skin layer differ substantially from patient to patient. According to a preferred embodiment, the residual puncturing depth is also settable—independently of the puncturing depth.
A further aspect of the invention which also has independent significance, relates to a puncturing instrument, in particular for a puncturing system according to any one of the preceding claims, comprising a lancet holder for receiving a sample collection unit and a drive for accelerating the lancet holder for a piercing and retraction movement, characterized in that the lancet holder is coupled to a damping mechanism to damp the deceleration of the lancet holder toward the end of a retraction movement.
By means of a damping mechanism according to the invention, the drive movement may be slowly stopped at the end of a retraction movement, so that painful oscillations of the piercing element may be reliably avoided. This is especially important if, at the time of stopping the movement, the piercing element projects by a residual puncturing depth into the skin of the patient.